F5 bias problems

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Hi all,

I'm having problems biasing one channel:
P2 does not affect bias !
When I measure R across R3 & 4 resistance drifts
I've changed Q4 and the jfets, I've changed the 5K trim pot , I've checked all connections and they are all well.

Is there anything else to look for- please…


Joined 2010
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Yes, Please take good photos and post them.

Question - what effect does P1 have? I ask only because you mention that P2 doesn't do anything.

Are you sure you have no cold solder joints on the PCB? That seems to be the Occam's Razor answer...
If you really get stuck trying to bias the F5 -- unsolder the source leg of the mosfets, take a DVM and measure the voltage across the drain resistor of each input JFET. From the manual you'll see that this voltage should be ~4.2V -- set it to ~3.5V, Oftentimes it will be too low or too high. Then you solder the MOSFET back in, and gradually adjust the trimpots for bias current and offset. It requires a bit of patience.

Remember that P1 is in parallel with R3 AND R15+TH1. (from the original schematic). If you can't get 4.2V with a lower IDss/gm JFET like a 2N5457, you may wish to consider changing R3 to a higher value, or placing another JFET in parallel with the original.
The most frequent issue I have heard of with F5 is caused by people trying to adjust only one of the pots first. Both pots need to be adjusted half turns at a time with constant monitoring of the mosfet source resistor voltages.
Resistance drifts due to the active components and capacitance, so focus on the mosfet source resistor voltages.
Thank you all for helping out!

Here is what I've got so far:

1-When measuring R12 (0.47 ohm) it starts at 1.5 ohm and quickly drops to 0.9- 1.0 ohm
2- When measuring R between outer legs of both trim pots resistance shifts up and is not stable.
3-When measuring V between outer legs of mosfet: Q3= 1.4 MV Q4= 9.5 MV
4-I've adjusted both trim pots 1 turn at a time and still P2 has no effect on both bias and offset
5-I'm not using a speaker protect circuit
6- the LED that should show power is on, does not turn on
1.- 0,47 is typically a challenge for normal multimeters, so don't worry about that.
2 - as explained before
3 - i don't think you get much useful from measuring there - you could try referencing to gnd instead
4 - could be many things
5 - OK
6 - so that is at least a very tangible issue you could try to resolve.

But I would restate what Zen Mod said - post pictures
did you check your PSU? do you have +/-23-25V? if you dont have the +23-25V then the LED will not light up.
and post the schematic that you use. there is many differt ones out there. so R12,Q3 don't tell us anything without knowing wich schematic you'r using.
Last edited:
Hello good people

Just for future reference if this should happen to anyone else!
Problem solved! It turn out I used 47K ohms instead of just 47 ohms in R 13 & 14 (what an idiot)

Thank you all for trying to help out and offcours thank you Mr. Pass for such a beautiful sounding amp !

Just one short question, I'm using a shielded power cable with 3X 18 awg wirers

Is 18 awg too wimpy for a power amp? Does it matter which side of the cable the shield is connected?
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